Is there a way to subscribe to a topic without setting the type?

edit

A version using a callback to make it a drop-in replacement for a rospy.Subscriber(). Based on KindDragon's answer.

Usage:

my_subscriber = GenericMessageSubscriber('topic_name', message_callback)

def message_callback(data):
    # handle your callback exactly as you would do it with a normal Subscriber
    print(data)

Class:

class GenericMessageSubscriber(object):
def __init__(self, topic_name, callback):
    self._binary_sub = rospy.Subscriber(
        topic_name, rospy.AnyMsg, self.generic_message_callback)
    self._callback = callback

def generic_message_callback(self, data):
    assert sys.version_info >= (2,7) #import_module's syntax needs 2.7
    connection_header =  data._connection_header['type'].split('/')
    ros_pkg = connection_header[0] + '.msg'
    msg_type = connection_header[1]
    msg_class = getattr(import_module(ros_pkg), msg_type)
    msg = msg_class().deserialize(data._buff)
    self._callback(msg)

import rospy import sys

from importlib import import_module

class Listener(object):
    def __init__(self):
        self._binary_sub = rospy.Subscriber(
            'some_topic', rospy.AnyMsg, self.topic_callback)

    def topic_callback(self, data):
        assert sys.version_info >= (2,7) #import_module's syntax needs 2.7
        connection_header =  data._connection_header['type'].split('/')
        ros_pkg = connection_header[0] + '.msg'
        msg_type = connection_header[1]
        print 'Message type detected as ' + msg_type
        msg_class = getattr(import_module(ros_pkg), msg_type)
        msg = msg_class().deserialize(data._buff)

        print data.known_field

Based on article http://schulz-m.github.io/2016/07/18/...

For your information, there is a package in C++ that does exactly this:

https://github.com/facontidavide/ros_...

piyushk is correct, but according to the wiki, "rostopic is a stable command-line tool within the ROS core toolchain. The underlying code may undergo refactoring for easier library use, but the external API is expected to be fairly stable."

Using functions like:

get_topic_type

data_type = rostopic.get_topic_type('my_topic', blocking=False)[0]
if data_type:
      data_class = roslib.message.get_message_class(data_type)
      #... do fun things here

Now, according to REP100 roslib.message is supposed to be moved to rosgraph but I can't find it there, even in fuerte.

I hope that this was really a reference to something else.. because there is sooo much code that is still using roslib.message...